Tag: idea of energy

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Multiple choice physics energy and its forms idea of energy introduction to work and energy work and energy

A window whose area is $2m^2$ opens on street where the street noise result in an intensity level at the window of $60db$. How much acoustic power enters the window via sound waves ? Now, if an acoustic absorber is fitted at the window? 

  1. $8\mu W$
  2. $10\mu W$
  3. $4\mu W$
  4. $2\mu W$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Intensity = 60 dB

60 = 10 log $(\frac{I}{I _{ref}})$

where $I _{ref}$ is the refrence intensity , general value of this is given by 

$I _{ref} = 10^{-12}\frac{watt}{m^2}$

$\frac{I}{10^{-12}}$ = anti log (6)

$I = 10^{-12}  \times{anti log(6)} = 10^{-12} \times{10^6} = 10^{-6}$

Power = I$\times$ area = $2 \times{10^{-6}}$ watt

Multiple choice physics energy and its forms idea of energy introduction to work and energy work and energy

The internal energy change when a system goes from state A to B is $40$ kJ/mole. If the system goes from A to B by a reversible path and returns to state A by an irreversible path what would be the net change in internal energy?

  1. $40$ kJ
  2. $> 40$ kJ
  3. $< 40$ kJ

  4. Zero
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation
For a cyclic process the net change in the internal energy is zero because the change in internal energy doesnot depend on the path.